The thin film of, for example, silicon oxide (SiO2) or silicon nitride (e.g., Si3N4) is widely used in semiconductor elements such as a thin-film transistor, photoelectric transduction elements, etc. Mainly, the following three techniques are employed for forming the thin film of, for example, silicon oxide or silicon nitride:
(1) physical vapor-phase film forming technique such as sputtering or vacuum vapor deposition,
which is a method wherein a solid thin film material is converted to atoms or atomic groups by physical means and deposited on a surface for film formation to thereby form a thin film;
(2) thermal CVD technique,
which is a method wherein a gaseous thin film material is heated to high temperature so as to induce chemical reaction, thereby forming a thin film; and
(3) plasma CVD technique,
which is a method wherein a gaseous thin film material is converted to plasma so as to induce chemical reaction, thereby forming a thin film.
In particular, the plasma CVD (plasma enhanced chemical vapor deposition) technique (3) enables efficiently forming a dense uniform thin film, so that it is now widely employed.
The plasma CVD apparatus 100 for use in this plasma CVD technique generally has the structure of FIG. 5.
Specifically, the plasma CVD apparatus 100 includes reaction chamber 102 in which a decompression state is maintained. The reaction chamber 102 in its interior is provided with upper electrode 104 and lower electrode 106 arranged opposite to each other with a given spacing. Raw-material gas supply path 108 connected to a raw-material gas source (not shown) is connected to the upper electrode 104 so that raw-material gas is fed into the reaction chamber 102 through the upper electrode 104.
Further, high frequency applicator 110 for applying high frequency power is connected to the reaction chamber 102 at the vicinity of the upper electrode 104. Moreover, exhaust-gas path 114 for discharging exhaust gas through pump 112 is connected to the reaction chamber 102.
In the use of the thus constructed plasma CVD apparatus 100, for example, where a thin film of silicon oxide (SiO2) is formed, monosilane (SiH4), N2O, N2, O2, Ar, etc. are introduced into the reaction chamber 102 in which a decompression of, for example, 130 Pa is maintained, through the raw-material gas supply path 108 and the upper electrode 104. In this plasma CVD apparatus 100, for example, where a thin film of silicon nitride (e.g., Si3N4) is formed, monosilane (SiH4), NH3, N2, O2, Ar, etc. are mainly introduced into the reaction chamber 102. During the introduction, a high frequency power of, for example, 13.56 MHz is applied between the upper and lower electrodes 104, 106 arranged opposite to each other in the reaction chamber 102 by means of the high frequency applicator 110. As a result, a high frequency electric field is generated, and, electron is impacted with neutral molecules of raw-material gas so that a high frequency plasma is generated in the electric field. In the high frequency plasma, the raw-material gas is dissociated into ions and radicals. The plasma CVD apparatus is so constructed that, by virtue of the reaction between ions and/or radicals and other materials, a thin film of silicon compounds is formed on a surface of semiconductor article W such as a silicon wafer disposed on one of the electrodes (lower electrode 106).
In this plasma CVD apparatus 100, at the stage of film formation, electric discharge within the reaction chamber 102 would cause a thin film material such as SiO2 or Si3N4 to adhere to and deposit on surfaces other than the surface of semiconductor article W on which film formation is to be conducted, for example, those of the inner wall of reaction chamber 102 and the electrodes, so that by-products would occur. When such by-products grow to a certain thickness, the by-products would be detached by the self weight thereof or a stress caused thereby. The detachment of by-products, at the time of film formation, would cause mixing of fine particles as foreign matter into semiconductor products and hence contamination of the semiconductor products. Therefore, this plasma CVD apparatus 100 cannot produce a thin film of high quality and has had the danger of causing breakage of semiconductor circuits or shortcircuiting thereof and also causing, for example, yield drop.
Accordingly, with respect to the plasma CVD apparatus 100, it is common practice to remove by-products with the use of a cleaning gas of, for example, a fluorocompound such as CF4, C2F6 or NF3 optionally loaded with O2, etc. after the completion of film formation so that the above by-products can be drawn off on occasion.
That is to say, in the method of cleaning by means of the conventional plasma CVD apparatus 100 using the above cleaning gas, as shown in FIG. 5, a cleaning gas consisting of a fluorocompound such as CF4, C2F6 or NF3, entrained by a gas of, for example, O2 and/or Ar, in place of the raw-material gas at film formation is introduced into the reaction chamber 102 in which a decompression state is maintained, through the raw-material gas supply path 108 and the upper electrode 104, after the completion of film formation step. In the same manner as in the stage of film formation, a high frequency power is applied between the upper and lower electrodes 104, 106 arranged opposite to each other in the reaction chamber 102 by means of the high frequency applicator 110. As a result, a high frequency electric field is generated, and electron is impacted with neutral molecules of cleaning gas so that a high frequency plasma is generated in the electric field. In the high frequency plasma, the cleaning gas is dissociated into ions and radicals. The resultant ions and radicals react with by-products such as SiO2 and Si3N4 adhered to and deposited on surfaces of the inner wall, electrodes and other parts of the reaction chamber 102, so that the by-products are gasified into SiF4. The gasified product together with exhaust gas is discharged outside the reaction chamber 102 through the exhaust path 114 by means of the pump 112.
However, the above fluorocompound such as CF4, C2F6 or NF3 used as the cleaning gas is a stable compound having a long life in the atmospheric air, and hence has a drawback in that to make the disposal of waste gas after the cleaning harmless is difficult, so that disposal cost is unfavorably increasing. Further, as the global warming factors (value for a cumulative period of 100 years) of CF4, C2F6, SF6 and NF3 are 6500, 9200, 23900 and 8000, respectively, they are extremely large. Therefore, the adverse influence thereof on environment is apprehended.
Namely, the conventional cleaning method for plasma CVD apparatus wherein the cleaning gas is introduced into the reaction chamber 102 in which a decompression state is maintained, through the raw-material gas supply path 108 and the upper electrode 104 and the cleaning gas is converted to plasma between the upper and lower electrodes 104, 106, is known as the “parallel plate type plasma CVD cleaning method”. In this method, the ratio of gas discharged outside the reaction chamber 102 through the exhaust path 114 as shown in FIG. 5 is so high that, currently, it has an adverse influence on global warming and also both the dissociation efficiency and cleaning capacity are low.
As a countermeasure, there has been proposed a cleaning method wherein, as shown in FIG. 6, NF3 is used as a cleaning gas and NF3 is introduced into remote plasma generator 101 disposed outside the reaction chamber 102. As a result, the NF3 is converted to plasma, and the cleaning gas of NF3 having been converted to plasma is introduced into the reaction chamber 102 in which a decompression state is maintained, through the raw-material gas supply path 108 and the upper electrode 104. Consequently, the surfaces of the inner wall, electrodes and other parts of the reaction chamber 102 are cleaned.
However, this NF3 gas has high toxicity, the influence on an environment is large and moreover it is expensive, so that cost of a semiconductor product is unfavoraable increased.
Under these circumstances, it is an object of the present invention to provide a cleaning method for CVD apparatus wherein the by-products such as SiO2 and Si3N4 adhered to and deposited on surfaces of the inner wall, electrodes and other parts of the reaction chamber at the stage of film formation can be removed efficiently, and wherein the discharge amount of cleaning gas is so small, the influence on environment such as global warming is little, and also cost reduction can be attained. Another object of the present invention is to provide a cleaning device for CVD apparatus used in such a method.